1. Field of the Invention
The present invention is directed to an apparatus for the intracardial acquisition of a measured signal corresponding to the physical activity of a subject, and in particular to an implantable apparatus for in vivo acquisition of such a signal.
2. Description of the Prior Art
Measuring instruments are required, for example, in cardiac pacemaker technology, for acquiring a measured signal corresponding to the physical activity of the subject in whom the pacemaker is implanted, for the purpose of adapting the rate with which the heart of the subject is stimulated to the physical activity of the subject. An apparatus of this type is described, for example, in European Application 0 249 822. This known measuring instrument includes means for forming a signal component corresponding to the blood oxygen content of the pacemaker user, by means of a light transmitter for emitting light into the blood of the user, and a light receiver for receiving the light reflected at the blood. The instrument also includes a temperature sensor for forming a signal component corresponding to the blood temperature. The instrument also includes a power supply circuit. Because the blood oxygen content proceeds to a saturation condition at a certain intensity of physical activity, the signal component corresponding to the blood oxygen content can only describe the physical activity of the subject with sufficient precision when the physical activity is at a relatively low intensity. By contrast, the body or blood temperature in the presence of physical activity represents a substantially exact indicator for the intensity of the physical activity. In the aforementioned known measuring instrument, the signal component corresponding to the blood oxygen content and the signal component corresponding to the blood temperature are combined (overlaid) in such a manner that a measured signal is obtained which can be reliably evaluated even with a high intensity of physical activity.
For the reason, the aforementioned known measuring instrument has an advantage over other known measuring instruments which supply either a signal corresponding only to the blood oxygen content or a signal corresponding only to the blood temperature. These other known measuring instruments responding only to the blood oxygen content or only to the blood temperature, however, also have advantages. It is possible to integrate the light transmitter, the light receiver and a temperature sensor in a catheter to be introduced into the heart of a subject, or to integrate those components into the electrode of a heart pacemaker. This does not present space problems because the light transmitter and the light receiver and the temperature sensor are extremely small components, however, the catheter must contain a line having at least four poles, or an electrode having at least four conductors must be used. It is also possible to integrate the drive circuit into the catheter or into the electrode, and for this purpose it is only necessary that the catheter contain a two-pole line or that the electrode have two conductors, however, integration of the drive circuit into the catheter or into an electrode in combination with the other components presents space problems.